This invention relates to an apparatus for producing and detecting induced heat radiation with a source of excitation radiation whose output radiation can strike an object, with the heat radiation induced by the source of excitation radiation being detectable by a detector, and with a beam deflector located in the beam path between the source of excitation radiation, the object, and the detector, with the output radiation and the heat radiation proceeding collinearly in a superimposed section between the object and the beam deflector, and with the beam deflector having at least one reflector element connected to a drive unit that limits the superimposed section with the drive unit.
Such a device is disclosed by DE-OS 23 00 436. In this device, there is a tilting mirror inclined with respect to a pulsed and focussed output beam of a laser as the source of excitation radiation, by which on the one hand the output beam can be directed to an object to be examined, and on the other hand, after tilting, infrared radiation collinear with the output beam emitted by the object from the irradiated area can be directed to a detector.
Tests can, in fact, be made with this device on the object with regard to heat dissipation as a reaction to the pulsed, focussed irradiation, but this method is unsuitable for serial tests in industrial quality control because of the necessity of maintaining the tilt angle precisely relative to the output beam, on the one hand, and on the other hand relative to the detector, and the resultant susceptibility to errors. Furthermore, additional devices have to be provided to keep the output beam away from the moving mirror when detecting the infrared radiation to prevent irradiation of the highly sensitive detector with stray scattered radiation. It is also a result of the tipping process that the end positions with the precise angles to be reached can be firmly assumed only after a relatively long period of time.
A device for separating and recombining optical radiation is disclosed by DE-PS 12 91 533, in which precisely reproducible dark pauses of equal length can be produced in two arms with a comparison substance and a test substance independently of the radiation path length, by means of rotating reflector disks with reflective, absorbing, and transparent sectors that have different sector angles. Certain tolerances are permissible here in the relative phase of the reflector disks and the congruence between the beam cross sections in the separating and combining regions, without giving rise to erroneous measurements.
A method and a device for controlling the production of welded seams are disclosed by GB 1 484 181, in which infrared radiation emitted from the two sides of the applied welded seam is directed in two focused beams to a detector by a sector wheel with reflective and perforated sectors. This is intended for reliable monitoring of the quality of the welded seam.
DE 40 15 893 A1 discloses a device for contactless and nondestructive examination of the internal and/or external structure of absorbing test specimens, in which induced temperature modulation can be produced locally with an intensity-modulated excitation beam. The back-emitted infrared radiation is separable from the excitation radiation by passing through a dichroic beam splitter that is also impacted by the excitation beam, and is diverted to a detector. It can in fact be determined with this device whether detected effects come from the interior or the surface of the material, but there is a high proportion of scattered light especially when evaluating close to the wavelength of the excitation radiation, which is detrimental to the signal/noise ratio and prevents measurements of weak signals.
DE 43 43 076 A1 discloses a device for the photothermal testing of a surface, in which excitation radiation falls on a surface to be tested through an opening in a focusing lens. The back-emitted heat radiation is fed to a detector through the focusing lens. Because of the separation of excitation radiation and heat radiation with no common optical elements, the individual components can be matched optimally to the individual wavelengths and in particular, back-emitted heat radiation can be evaluated even in the spectral region close to the excitation radiation without optical elements impacted by both radiations leading to superimposition of the measured signal and the excitation radiation, in which case either the excitation radiation would strike the detector with substantial intensity or the heat radiation would be prevented from reaching the detector. This device, however, has the drawback that the sizes of the optical elements have to be matched precisely to one another for a given beam geometry to produce minimal losses through the opening in the focusing lens.
The underlying purpose of this invention is to provide a device of the type mentioned initially that is stable in its optical precision in fast serial tests and is not susceptible to problems.